The Coffin Fly

May 22, 2014

Photo by Kelsey Bailey.
As you well know, we are fly obsessed here at BioSCAN. Particularly, we are phorid obsessed. I am particularly obsessed with the macabre species Conicera tibialis, commonly known as the Coffin Fly. Perhaps it's the shadowy lighting as I view them under the microscope, but these flies, with their dark velvety bodies and (almost sinister looking) conical antennae (males only, females have round antennae), appeal to me tremendously.

Photo by Kelsey Bailey
A number of phorid species are known to colonize humans remains, but C. tibialis seems the most determined. Adult females of this species are known to dig down through over 2 meters of dirt and enter coffins to lay their eggs. To complete an equivalent journey, a human being would have to dig 2 miles down — in perspective the feat seems all the more remarkable! Once the females reach the corpse they lay their eggs on, or near, the cadaver. The maggots hatch and feed on the decaying tissue — they are known to prefer lean tissue (while other taxa, such as some species of beetles, prefer adipose tissue). Yes, even corpse eaters can be picky! C. tibialis is known to be able to cycle multiple generations without surfacing (what they are doing below ground, the living can only imagine!). When the flies do surface, they do so by crawling the reverse path of their ancestors: back up through many feet of dirt. Charles Colyer, in a paper from 1954, conveyed the observations of his friend Mr. R.L. Coe that were some of the first key insights into the life history of this species. In May of that year, Mr. Coe observed a number of C. tibialis running about a patch of his garden, where 18 months before he had buried his deceased dog. As Mr. Coe observed more closely, he realized that all the running about was actually a mating frenzy — complete with pairs frolicking in coitu! On Colyer's request, Coe dug down to the corpse of his former pet, observing phorids at every depth along the way. The flies were all traveling toward the surface, in a mass exodus from the grave — hoping to join the mating party so that they might return to this, or another, grave and lay eggs of their own. Alas, Mr. Coe reported that by June 16 the phorids could no longer be found in their mating frenzy in his garden — where for weeks they had been seen "running over the ground in sunshine, and congregating under loose clods of earth in inclement weather".

Photo by Kelsey Bailey.
C. tibialis is known not only to dig to astounding depths for corpses, but to wait unbelievably long periods of time to colonize. Corpses are typically utilized over a year after burial, and a paper by Martin-Vega et al. (2011) revealed a case where the species was found breeding on human remains 18 YEARS postmortem. Phorid species are some of the key insects used in the field of forensic entomology — a branch of forensics utilizing insect life cycles to help approximate the age of a corpse. A story detailing the occurrence of C. tibialis in California was recounted by Father Thomas Borgmeier (1969), one of the "fathers" of phoridology. He was sent specimens of the fly that were collected in a mausoleum in Colma. A family had constructed an above-ground resting place for their deceased in 1962, and in 1965 noticed large numbers of the flies both in the mausoleum and around the cemetery. The family made the decision to open the four crypts. All four crypt interiors were dry and filled completely with C. tibialis and spiders. I bet the phorids were happy they had such easy colonization - no digging required! The take home message of this macabre tale should not be one of disgust. Although the details may be gruesome, insects that colonize corpses are performing the necessary breakdown of organic material that must occur postmortem. Only by this breakdown — by insects, fungi, and bacteria — can bodies be released to reenter the circle of life. Consumers of carrion are beneficial, performing an invaluable service to us below the surface. BioSCAN Principal Investigator Brian Brown likes to say that being food for C. tibialis is one way we can all contribute to the well being of phorids. I hope you might be as delighted by this fly as I am after reading the amazing life history and marveling at the amazing photos taken by our star photographer Kelsey Bailey, who expertly capture the dark, sleek aesthetic of this species on film (well... digitally [did I just date myself?]). When I handed Kelsey a vial with several dried specimens, I told her I wanted creative photos to visually express the morbid life history of these flies. As you can see, she did not disappoint. I particularly enjoy the photograph at the top of the post — Kelsey beautifully mounted the specimen on the head of an insect pin — a glittery orb I wish appeared more often in entomological photos. I also like the film noir feel of the "portrait" she took of this species. Yes, I really like this fly. Perhaps my love for C. tibialis is so deep (2 meters, to be exact) because I know they will be with me not just in life, but for up to 18 years past my death. VITA INCERTA, MORS CERTISSIMA.

Since people are the only animals to bury their dead & coffin flies have this remarkable ability to dig into grave sites, has their evolution coincided with that of human burial practices? Can coffin flies successfully compete for carrion exposed in the wild - or are they largely dependent on this peculiar human pastime?

Hi Walter! As usual, great questions! C. tibialis cannot outcompete other insects (for instance, flies from the family Calliphoridae) at the surface. The surface actually presents a number of problems, including many types of vertebrate carrion eaters that would disrupt potential colonization. Coffin flies specifically outcompete other insects for bodies either under the surface (or otherwise hard to reach, such as in a sealed casket in a mausoleum) and after long periods of time. Other insects will appear first, and C. tibialis will not colonize a cadaver until at least one year postmortem, more typically 3-5 years. The different insects appear in succession as breakdown occurs. Although there are few examples of other animals that cover their dead with leaves and essentially "bury" them, quite a few animals end up "buried" because they are burrowing animals and die underground. Additionally, one might assume (this is pure conjecture on my part) that after several years most corpses may be partially or even wholly covered, based on location and circumstance (including parts that are carried off by vertebrates and buried). This is the point at which C. tibialis would thrive. Insects having been around so much longer than humans, our burial habits certainly made their life history more convenient, but did not shape its evolution.

Thanks Emily! I will have to defer to you on all things "fly." And I understand the other incidental burial scenarios that could create opportunities for coffin flies to thrive in the wild...but is it fair to dismiss the potentiality of human settlement affecting insect evolution? True that insects have been around long before humans, but did C. tibialis in its current form actually pre-date humans? Even if it did, bugs iterate thousands of times within the lifespan of a single human. Surely there must be at least one insect out there that has evolved as a discrete species as a result of human settlement. There are domesticated insects like silk worms & honey bees - but that is too easy. There has got to be a bug out there that has evolved "by accident" as a result of pressures or opportunities created by human civilization. Maybe its the bed bug or the cheese mite! It's out there! I know it is!! (No more coffee for me)

Hi Walter! Genetic shift happen within species with every generation, so I am certainly not ruling out some pressure on the genome of C. tibialis over the length of human history. The extent of these shifts may be relatively modest with regards to speciation. Anna Holden, a colleague here at the museum, recently found an extant species of bee preserved in the La Brea Tarpits. Clearly, if that bee was in its modern form millions upon millions of years ago, the possibility exists that C. tibialis (and many other insects) are in similar situations. To be honest, I'm not well versed in this particular area — I will have to remember to talk to Brian Brown about this type of thing. He has done quite a bit of work with flies preserved in amber and may have some good insights.

I buried my cat in a water “resistant “ casket with a lid that screwed on on all four corners. I am now thinking of exhuming his body and having whatever remains cremated. What can I expect to find inside? I own my house but know that I probably can’t stay here forever and the thought of having to leave him, destroys me.

The author responds:
Hi Melissa! I am sorry for this delayed response. It is very difficult to know what you will find if you decide to exhume your cat's remains. The biggest factor here will be time since burial. As far as moisture, even coffins sold for people that are supposed to be watertight are unlikely to remain so forever, so a pet coffin would be the same. How much moisture has entered the coffin depends on the moisture at the burial site in addition to the construction of the coffin. Coffin flies, and other decomposers, would actually be of benefit if they entered the coffin (and they are very good at entering coffins)! The role of decomposers is to "clean up" decaying organic material, so the best case may be that they have done their job and the remains are dry and mostly bones, as in the case discussed in this blog from Colma. You could then cremate the dried remains. If the burial took place some years ago and is in a relatively dry area, it is reasonably likely nature will have taken its course and this is what you will find. I wish you the best with this endeavor, as a cat owner myself I understand how difficult the idea of leaving your cat's remains must be! -- Emily

I ran across your article after I visited the Johnson Island war cemetery in Ohio where I noticed large amounts of what I thought were gnats located all over ever section of the cemetery that had grave markers. I noticed when I went to one end of the cemetery where no grave marks where there were no fly's. This cemetery had its last burial in 1865 yet today there are swarms of fly's over the graves. I found it rather odd to see this occurrence so many years after the last burial.